1. Field of the Invention
The present invention relates to thermosettable compositions for composites, and more specifically, the present invention relates to thermosettable compositions for composites and electrical laminates; and the composites and electrical laminates made from the thermosettable compositions.
2. Description of Background and Related Art
For electrical laminates and composites, thermosets that do not change properties with changing environmental conditions, such as humidity and temperature, are very desirable. Three desirable properties for aryl glycidyl ethers for these applications are high glass transition temperature (Tg greater than 190° C. by dynamic mechanical thermal analysis with dicyandiamide cure), low monomer viscosity (less than 200 mPa-s at 150° C.), and high epoxy equivalent weight. Epoxies of the present invention exhibit viscosities as low as 120 mPa-s and the dicyandiamide-cured thermosets have Tgs up to 202° C. and EEWs (epoxy equivalent weights) of greater than 190 grams/equivalent (g/eq).
High Tgs are needed for applications where the composite will be exposed to high temperature, for example in a printed circuit board that cycles between ambient and high temperature many times during its lifetime. The properties of the composite degrade precipitously above the temperature of the Tg, and in general high Tg's offer a wide range of use temperature.
A variety of methods are used to prepare composites, and low viscosity resins are typically a necessity. For example, a common process for composite part fabrication is to enfuse a mold that contains a fiber preform. If the resin viscosity is too high the fiber perform will be deformed. Low viscosity has other benefits. For example, adhesion to fiber is usually better for low viscosity resins because wetting is improved and the resin can intercalate into fiber bundles.
Finally, epoxy resins with high EEWs give thermosets with a relatively low concentration of hydroxyls in the backbone. Hydroxyls are formed during typically curing reactions of epoxy resins, such as with dicyandiamide, a multifunctional amine. There is a direct relationship between hydroxyl concentration in a thermoset and water absorption. High water absorption is undesirable in especially in electrical laminates because the properties such as dielectric performance, change with changing humidity.
There are many aryl glycidyl ethers that achieve these properties individually, but not that meet them all properties simultaneously. This balance of properties is difficult to achieve. For example, one common strategy for high Tg is to use polyglycidyl ethers of highly functional polyphenols, especially phenol formaldehyde novolacs which are known as epoxy novolacs. However, examples of such novolacs that have viscosities of less than 200 mPa-s are not capable of achieving high Tgs comparable to the epoxy resins of the present invention. For example, D.E.N.™ 438, an industry standard epoxy novolac, has a viscosity of <200 mPa-s but the Tg of the dicyandiamide-cured thermoset is only 173° C.
Accordingly, there is still a need in the industry to develop new thermoset resins useful for coatings that are difunctional and provide thermosets with a balance of properties including high Tg (>150° C.), low monomer viscosity (<150 mPa-s at 150° C.) and high EEW (>190 g/eq).